import numpy as np
import matplotlib.pyplot as plt

# a = np.linspace(0, 2 * np.pi, 50)
# b = np.sin(a)
# print(a)
# print(b)
# plt.plot(a, b)
# mask = b >= 0
# plt.plot(a[mask], b[mask], 'bo')
# mask = (b >= 0) & (a <= np.pi / 2)
# plt.plot(a[mask], b[mask], 'go')
# plt.show()

a = np.arange(0, 100, 10)
b = np.where(a < 50)
c = np.where(a >= 50)[0]
print(a)
print(b)
print(c)
# [ 0 10 20 30 40 50 60 70 80 90]
# (array([0, 1, 2, 3, 4], dtype=int64),)
# [5 6 7 8 9]
